One of the enduring mysteries in physics is high critical-temperature superconductivity—or high-Tc superconductivity. All superconductors (materials that conduct electricity with no resistance) require very low temperatures compared with room temperature, but the high-Tc superconductors have transition temperatures that are higher than their conventional cousins—30° to 110° Celsius above absolute zero, compared to a few degrees. This slight bump in allowable temperatures makes high-Tc superconductors a bit more reachable experimentally, but exactly how they conduct electricity is still mysterious.

A new study published in Science examined a particular class of high-Tc superconductor, known as an iron pnictide. ("Pnictide" refers to an atom in the same column as nitrogen in the periodic table.) K. Hashimoto et al. found evidence of a quantum critical point (QCP): a place where the material's properties change radically due to quantum fluctuations rather than changes in temperature or pressure. While many physicists suspect the presence of a QCP in high-Tc superconductors, none have found unambiguous evidence for its existence. The current study is still not definitive, but the particular iron pnictide material the researchers used provides far cleaner data—and stronger hints that the QCP is actually there. Its presence would reveal a great deal about the inner workings of high-Tc superconductors, perhaps helping lead to even higher temperature superconducting devices.

The phases of matter many of us learned in school—solid, liquid, and gas—are based on the ordering of atoms within materials. Additional phases, including superconductivity, are explicitly quantum in character, relying as they do on the ordering of electric charge carriers. The properties of these charge carriers arise from interactions rather than being fundamental particles like electrons, but they still act like particles: they may have mass, charge, and spin. Materials can change from one phase to another when the temperature or pressure is changed, though the superconducting phase transition can also be induced by introducing atoms whose electrical properties provide extra charge carriers. This process is called doping.

In the case of high-Tc superconductors, the key parameters are temperature and doping. The iron pnictide superconductor in the recent study was BaFe2(As1-xPx)2, where "x" is the doping fraction. (In this case, the pnictide is the arsenic.) The researchers picked this particular pnictide due to the ease with which pure crystals of the material can be grown and how clean the resulting data is. For x values roughly between 0.2 and 0.7, BaFe2(As1-xPx)2 is a superconductor; outside those values, the material isn't superconducting at any temperature.

A QCP—if it is present—marks another type of phase transition, where quantum fluctuations at absolute zero change the superconducting behavior of the material. While absolute zero isn't experimentally achievable, the quantum fluctuations start at (relatively) higher temperatures, changing the behavior of the flow of the charge carriers. One measure of the flow is known as the London penetration depth. (This quantity actually determines how far magnetic fields can penetrate into the superconductor. An ideal superconductor repels all magnetic fields.) Near the hypothetical QCP, the penetration depth grows to large values.

The researchers found that at the optimal doping value, the London penetration depth jumped sharply. While this behavior doesn't automatically mean there is a QCP, it's certainly suggestive and could explain other phenomena in iron-based superconductors. A side effect of a QCP is to divide the superconducting behavior into two regions, based again on doping. In one of these superconducting phases, both superconductivity and magnetism may be able to coexist, a phenomenon not seen in other materials. Hunting for the telltale signs of this second phase transition could be a next step, as the authors stated.

If the QCP is actually present, it may be the driving factor for high-Tc superconductivity. The current study could prove to be significant progress toward solving the enigma of these materials.